Quench arrangement

ABSTRACT

Quench arrangement for use in the cracking of hydrocarbons, which arrangement includes a precooler followed by a pair of aftercoolers connected in parallel.

United States Patent Inventor Hartoch J. Cljer The Hague, Netherlands 793,523

Jan. 23, 1969 Sept. 21, 1971 Selas Corporation of America Sept. 20, 1968 Netherlands Appl. No. Filed Patented Assignee Priority QUENCH ARRANGEMENT 6 Claims, 2 Drawing Figs.

US. Cl 48/102 R, 48/102 A, 48/211, 165/100, 165/101 1111.01 Cl0g9/04, F28f27/02 F leld of Search [56] References Cited UNITED STATES PATENTS 1,908,733 5/1933 Creel .L 165/101 3,031,287 4/1962 Bensonetal. 48/197 3,285,719 11/1966 BodleetaL'. 48/196X Primary Examiner-Morris O. Wolk Assistant Examiner-R. E. Serwin Attorney-E. Wellford Mason ABSTRACT: Quench arrangement for use in the cracking of hydrocarbons, which arrangement includes a precooler followed by a pair of aftercoolers connected in parallel.

PATENTEBSEFZI 1971 WATE R INVENTOR. HAR TOC H J. CIJFER /ZQ. W

QUENCI-I ARRANGEMENT SUMMARY OF THE INVENTION The present invention relates to apparatus useful in the crar 'ng of hydrocarbons having a high-ethylene content. The apparatus is of the type in which the effluent or product from the cracking furnace is passed directly to a precooler and an aftercooler, both of the indirectIy water-cooled type, and both used to generate high-pressure steam.

Hydrocarbons which are to be cracked to a product having a high-ethylene content are heated to 700-920 C. in a gaseous or vaporous state in a tube-heating furnace. Immediately after leaving the furnace the product gas has to be cooled. Cooling generally takes place in an indirect heat exchanger which is subject to fouling with this being most serious when heavier hydrocarbons are cracked.

In the cracking of hydrocarbons, a lighter feed stock must be heated to a higher temperature than a heavy feed stock, and has less tendency to coke and foul the system than does a heavy one. Because of this, more heat must be removed from the product gases produced from light hydrocarbon feed than must be removed from a heavy feed. In addition, the length of run for light feed, before decoking is required, is longer than for a heavy feed.

Operating periods, before cleaning or decoking of the heat exchanger is required, vary from 3 months for light naphtha (boiling range 50-l20 C.) to 6 weeks for heavy naphtha (boiling range l-200 C.) to about 2 weeks for gas oil (boiling range 200400 C.) Stopping the apparatus in order to clean or decoke the heat exchangers is an expensive and time-consuming process, and it was therefore believed to be economically impractical to crack gas oil to ethylene in presently used arrangement of apparatus.

It is an object of the invention to provide a quench arrangement which will make it commercially practical to crack gas oil into ethylene by increasing the length of run before decoking is required.

It is a further object of the invention to provide a quench arrangement which can be used to adequately cool various types of hydrocarbons as they are leaving a cracking furnace.

The objects of the invention are made possible by apparatus to be described which includes a precooler immediately adjacent to the outlet of the furnace, which is closely followed by a pair of aftercoolers that are connected in parallel so that one or both of them can be used, depending upon the hydrocarbon being cracked.

IN THE DRAWINGS FIG. 1 shows diagrammatically the apparatus of the invention, and

FIG. 2 shows in section the distribution cone of the precooler.

DETAILED DESCRIPTION As shown in FIG. 1, the feed of the hydrocarbon being cracked, which may be diluted with steam, is supplied through line 1 to furnace 2 where it is heated to cracking temperature. Furnace 2 may be of any conventional'type, capable of raising the feed to the desired temperature; such a furnace being shown in US. Pat. No. 3,353,920. The mixture of cracked gases from the furnace flows directly to a precooler 3 that is located immediately adjacent to the outlet of the furnace, as explained below. The outlet from precooler 3 is split with one branch going through a high-temperature valve 4 to an aftercooler 5, and the other branch going through a second hightemperature valve 6 to a second aftercooler 7. The outlets from the aftercoolers go through valves 8 and 9, respectively, to line 11 and further cooling and processing means. The arrangement is such that by adjustment of the valves the discharge from precooler 3 can be directed through either or both of the aftercoolers. Each of the coolers is a straight flowtube-type heat exchanger that is designed in a conventional manner for the amount of heat it is to extract from the product gases. This will depend upon the size of the installation. Each cooler is shown as being supplied with cooling water that may come from the same or different sources, and with a line through which steam produced in the cooler may be piped to a point of use.

Precooler 3 is a tube heat exchanger of conventional design except that its entrance end is designed to give a minimum of resistance to the flow of the fluid through it. The entrance section is shown in FIG. 2 as a truncated conical member 12 having a flange 13 by means of which it can be attached to the supply line from furnace 2, and a flange 14 by which it is connected to the shell of a tube heat exchanger. Product cooling tubes 15 extend fanlike in member 12 from the entrance to the outlet. The left ends of the tubes are close together and are held in position by a plate 16 across the entrance. The right ends of tubes 15 spread outwardly and are located in their separated positions by a spreader plate I7 in the large end of member 12. The right ends of tubes 15 are positioned and designed in such a way that they fit easily into the open ends of the heat exchanger tubes.

It is preferred that the precooler 3 be positioned as close as possible to the exit of the furnace so that the residence time of the cracked gas in the space between the furnace exit and the precooler exit is between 0.005 and 0.01 second, and preferably about 0.008 seconds. A precooler having an entrance cone the type shown has negligible fouling. This results from the very short residence time and consequent high velocity of the product gas which is in the order of l00-300 meters per second. The short residence time insures that the composition of the product gas will not be substantially altered after it leaves the furnace.

It is required that the precooler have a capacity such that the product gas temperature be lowered at least to the maximum operating temperature of the high temperature valves 4 and 6 at the inlet ends of aftercoolers 5 and 7. At the present time this maximum temperature is about 700 C. To obtain the required precooling, notwithstanding the short residence time, the precooler should have a high rate of heat transfer, in the order of 200,000 kcalJm /hour.

The inlet and outlet valves at the ends of the aftercoolers permit the use of either one or both of the aftercoolers at the same time. In the former case, the cooler not being used is completely disconnected from the process, and can, therefore, be cleaned without the process being shut down. The aftercoolers may be of conventional design so that they may be cleaned with high-pressure water jets. Fouling occurs rapidly in these coolers particularly when the temperature of the product is reduced below its dew point, in the order of 500 C. As noted above, both the precooler and the aftercoolers are water-cooled heat exchangers. They will generate high-pressure steam at from -130 kgJcm When the product gas leaves the aftercoolers it may be further cooled by direct heat exchange with a cooling medium to a temperature at which processing is practically feasible, as is usual in these processes.

The apparatus according to the invention is very well suited to handle a great variety of hydrocarbon feeds up to and including gas oil. The invention therefore concerns a process for cracking hydrocarbons or mixtures thereof with a boiling point up to 400 C. in apparatus such as described above. When a heavy feed is used, such as heavy naphtha or gas oil, only one aftercooler is employed while the other may be cleaned. If, however, a lighter feed is used such as light naphtha, both aftercoolers will be used in parallel because with this feed fouling is considerably less. In fact, the lighter feed is usually cracked at a higher temperature, so more heat has to be removed from the product gas. Thus, the apparatus is suitable for use with the cracking of both a light and a heavy feed.

A switch is made from one aftercooler to the other, first by opening the inlet and outlet valves to the cooler being placed in use, then closing the valves to the one having been used. The switch is made when there is an undesired increase in the pressure drop of decrease in temperature drop across the cooler. The drops that can be tolerated will depend upon a number of things such as the feed stock being cracked, the degree of cooling that is required, and the amount of fouling occurring.

To avoid fouling of the precooler it is desired to keep the outlet temperature thereof well above the dew point of the product gas. This outlet temperature will usually be in the range of 550700 C., and preferably about 650 C. when cracking gas oil. The outlet temperature of the aftercoolers will usually be in the range of 350-550 C., preferably about 450 C. Because of the further processing of the product gas, it is desirable that the pressure at the outlet of the aftercooler is not lower than 0.4 kg./cm The furnace outlet pressure, which is usually in the range of 0.8-1.2 kgJcm may therefore influence the allowable pressure drop over the system of precooler and aftercooler.

What is claimed is:

1. Apparatus for cracking hydrocarbons including in combination a furnace having a tube therein through which the hydrocarbons are passed, a precooler connected directly to said tube adjacent to the location where it leaves said furnace, a pair of aftercoolers located adjacent to the exit of said precooler, means including a valve to connect each aftercooler individually to said precooler, a pipe to carry the heated hydrocarbons from said aftercoolers for further processing, and means including a valve between each aftercooler and said pipe.

2. The combination of claim 1 in which said precooler is an indirect heat exchanger, means to supply cooling water thereto and means to remove steam therefrom.

3. The combination of claim 1 in which said aftercoolers are indirect heat exchangers, means to supply cooling water to each aftercooler and means to remove steam therefrom.

4. The combination of claim 1 in which the hydrocarbon is a light naphtha, and said valves in said means connecting said precooler and said aftercoolers are open, and the valves in said means connecting said aftercoolers and pipe are open.

5. The combination of claim 1 in which the hydrocarbon is a gas oil, and the valve in said means connecting said precooler and one aftercooler is closed and the valve in the means connecting said one aftercooler and pipe is closed.

6. Apparatus for cracking hydrocarbons including in combination a furnace having a tube therein through which the hydrocarbons are passed, a precooler connected directly to said tube adjacent to the location where it leaves said furnace, a pair of aftercoolers, means including a valve to connect after each aftercooler individually to said precooler, a pipe to carry the heated hydrocarbons from said aftercoolers for further processing, and means including a valve between each aftercooler and said pipe, said precooler being provided with an entrance casing in the form of a hollow truncated cone, a plurality of distributor tubes in said cone, means for bunching said tubes together and holding them in the small and of said cone, and means to hold the other end of said tubes in spaced relation at the large end of said cone. 

2. The combination of claim 1 in which said precooler is an indirect heat exchanger, means to supply cooling water thereto and means to remove steam therefrom.
 3. The combination of claim 1 in which said aftercoolers are indirect heat exchangers, means to supply cooling water to each aftercooler and means to remove steam therefrom.
 4. The combination of claim 1 in which the hydrocarbon is a light naphtha, and said valves in said means connecting said precooler and said aftercoolers are open, and the valves in said means connecting said aftercoolers and pipe are open.
 5. The combination of claim 1 in which the hydrocarbon is a gas oil, and the valve in said means connecting said precooler and one aftercooler is closed and the valve in the means connecting said one aftercooler and pipe is closed.
 6. Apparatus for cracking hydrocarbons including in combination a furnace having a tube therein through which the hydrocarbons are passed, a precooler connected directly to said tube adjacent to the location where it leaves said furnace, a pair of aftercoolers, means including a valve to connect after each aftercooler individually to said precooler, a pipe to carry the heated hydrocarbons from said aftercoolers for further processing, and means including a valve between each aftercooler and said pipe, said precooler being provided with an entrance casing in the form of a hollow truncated cone, a plurality of distributor tubes in said cone, means for bunching said tubes together and holDing them in the small end of said cone, and means to hold the other end of said tubes in spaced relation at the large end of said cone. 